It is very important to correctly transmit information including the colors of various goods, products, and works of art to the third party through a printed matter or the Internet. In general, in the printed matter, for example, an image is formed by color materials of four or more colors including cyan (C), magenta (M), and yellow (Y), which are the three primary colors in subtractive color mixing, and a black (K) ink.
In general offset printing, a printing plate, such as an aluminum plate with a smooth surface, is used. Ink is attached to a lipophilic printing element of the plate and dampening solution is attached to a hydrophilic non-printing element to prevent the ink from attaching. An ink supply roller and a dampening solution roller come into contact with a plate cylinder and ink is supplied following water. Therefore, ink is stably attached only to the printing element.
The ink supply roller is a mechanism configured by combining a plurality of rollers and ink is supplied from an ink fountain storing ink to the ink supply roller. Therefore, for example, a large amount of ink is used in a portion of the printed matter including a large area of images and a small amount of ink is used in a portion including a small area of images. In some cases, even when a constant amount of ink is supplied, the density of the color varies depending on images.
An ink key is provided between the ink fountain and the ink supply roller and controls width of the gap from the ink fountain. In addition, an ink fountain roller which comes into contact with the ink key is rotated at a constant rpm during printing to supply ink from the ink fountain. The number of rotations of the ink fountain roller is changed to control the flow rate of ink from the ink fountain to the ink supply roller for the entire printing surface. The number of ink keys varies depending on a printing press, and about 20 to 30 ink keys are generally provided. The flow rate of ink can be independently controlled.
In the printing process, in order to make a reference color proof, a proof output is performed several times by a printing press or an electronic printer. Until a desired color is reproduced, adjustment, such as color correction, is performed and a final contract proof is made.
In a case of proof, color reproduction is greatly changed depending on a proof manufacturer or the type of machine, in addition to the color of the color material of a proofer (color proof output device). In some cases, a change of color occurs in a printing environment due to, for example, a difference in the printing speed or the amount of dampening solution in a printing stage, even in a press proof on the printing press.
In the color matching of the printing press according to the related art, the operator visually compares a proof print or a proof sheet with a printed test sheet. The operator adjusts the amount of ink according to the color reproduction of the picture such that the color of the printed test sheet is close to the color of the final contract proof, based on the comparison result. However, this method greatly depends on the workmanship of the operator and the occurrence of a color difference is inevitable due to the differences among the operators.
A technique has been proposed in which a control strip, which is a kind of color chart including solid patches of C, M, Y, and K inks, is printed outside the image of the printed matter, the color of the control strip is measured by a color measurement means, such as a spectrophotometer, and the amount of ink supplied is managed based on the measurement result such that the density of ink is uniform.
However, in this case, since printing is a physical phenomenon, it is difficult to automate control for the ink fountain. In addition, since there is gradation or a background pattern in the color of a printing picture, which will be a product, it is difficult to stably measure the color with the color measurement device, as compared to the control strip. Furthermore, since there is no means for managing the color of the picture, the color adjustment of the picture of the printed matter depends on the experience or color sense of the printing operator or the color administrator.
In order to solve the above-mentioned problems, a technique has been proposed in which point color measurement (spot area color measurement) using a spot with a diameter of about 3 mm is performed to measure the color of an image and the measurement result is reflected in the amount of ink supplied. For example, Patent Document 1 discloses a technique which measures the color of a control point in a printed matter, controls the supply amount of color inks which are used at the control point based on the measured value, and controls the supply amount of other inks using a control strip.